To establish the efficacy of SB-207266-A (20mg, 5mg and 1mg od) in the treatment of Irritable Bowel Syndrome (IBS) over a 12 week treatment period. Secondly, to compare the dose/response relationship of SB-207266-A at doses of 20mg, 5mg, and 1mg per day. Lastly to compare the safety and tolerability of SB-207266-A (20mg, 5mg, and 1mg od) with placebo.